A day at the beach is incomplete without the sticky feel of sunscreen leaving white streaks across your cheeks. We wear it because we know we have to. Dermatologists and our mothers alike have instilled into our brains the need for a high SPF sunscreen since before we could walk. But what exactly does having a “high SPF” sunscreen entail?

The SPF on sunscreen stands for sun protection factor, a relative measurement for the amount of time the sunscreen will protect you from ultraviolet (UV) rays. UVB rays primarily affect the outer layer of the skin, the epidermis. They are responsible for sunburns and some surface-level skin cancers. The sun also emits UVA rays, which can penetrate into the lower level of the skin, called the dermis. UVA rays are typically associated with “tanning.” However, the darker color of the skin is a sign of damage to cells in the dermis. SPF numbers typically refer only to UVB rays, but some sunscreens can protect against UVA as well.

How does the SPF tell you how long you’re protected for? Well, let’s say you typically burn after being outside for 30 minutes and have an SPF 15 sunscreen you plan to use. You’d multiply 30 by the SPF, in this case 15. That means that you’re technically protected for 450 minutes, or 7.5 hours—do note that this is technically speaking. Most sunscreens will wear off long before then from exposure to the elements as well as improper application. It is recommended that you reapply every two hours. The number of UVB rays you’re protected from also increases with SPF, though marginally. SPF 15 blocks 93% of UVB rays, SPF 30 blocks 97%, and SPF 100 blocks 99%.